Flash memory devices may experience data retention issues. For example, data retention in a cell in a floating gate NAND memory may be compromised by charge loss at a floating gate in the cell. Charge may be lost through tunnels leading from the floating gate to a substrate. A flash cell's ability to hold onto its charge, and thus to accurately hold onto its programmed data value (e.g., 0, 1) varies inversely with the number of erase and program cycles experienced by the cell. In some examples, as few as seven erase and program cycles can degrade a flash cell where charge loss becomes significant enough to affect data retention capability.
Flash memory devices have programmed threshold voltage (Vt) distributions. When charge is lost in a cell, Vt distributions may deviate from the programmed value. If the Vt distribution deviation is sufficiently large, then data errors may appear in subsequent reads from the cell. Conventionally, some flash memory devices have accounted for charge loss related data retention issues by adjusting the read reference voltage for a cell. For example, the read reference voltage may be adjusted to track Vt distribution charge, as illustrated in prior art FIGS. 1, 2, and 3 to be described below.
Prior art FIG. 1 illustrates Vt distributions in a cell immediately after the cell is programmed. A first possible distribution 100 and a second possible distribution 110 are separated by a detection threshold 120. Distribution 100 and distribution 110 may represent the two possible values (e.g., 0, 1) that may be stored in a flash cell. Only one of the distributions would describe charge in a cell at a time. Since there is adequate separation between distribution 100 and distribution 110, and since neither distribution approaches the detection threshold 120, a correct value (e.g., 0, 1) would likely be readable from the cell.
Prior art FIG. 2 illustrates Vt distributions in a cell after a period of time and after a number of erase and program cycles. While first distribution 100 is substantially intact, second distribution 110 has degraded to distribution 130 due to charge lost from tunneling. Notice that distribution 130 is shifted and widened as compared to distribution 110. Distribution 130 has widened out so far that it has approached detection threshold 120. With this widening, it is possible that an incorrect value could be read from the cell. Thus, conventional systems may shift detection threshold 120. In some flash memories, a higher state distribution tends to shift more easily than a lower state distribution.
Prior art FIG. 3 illustrates the results of shifting detection threshold 120 to detection threshold 140 in response to the charge loss that produced shortened and widened distribution 130. While shifting detection threshold 120 to detection threshold 140 may temporarily alleviate some detection issues, this conventional approach may, over time, still yield some decoding failures. For example, decoding failures may occur when an excessive number of flash cells have experienced a charge loss beyond what can be accommodated by adjusting the detection threshold. One skilled in the art will appreciate that as distributions continue to widen and shift due to ongoing charge loss, that at some point there will be no location at which a detection threshold can be placed that will consistently produce accurate results. One skilled in the art will appreciate that information included in the background section is not admitted prior art.